The present invention relates to a member for developing electrostatic latent images for use in a development unit using a non-magnetic one-component toner.
As a development method using a non-magnetic one-component toner, a so-called contact development method is in general use, by which an electrostatic latent image bearing member is brought into contact with a toner transporting member for transporting toner particles charged to a predetermined polarity onto electrostatic latent images for developing the same to visible images.
The toner transporting member (hereinafter referred to as a development roller) is required to have many functions and to meet, for instance, the following requirements: When the electrostatic latent image bearing member is made of a rigid material, the development roller to be used with the electrostatic latent image bearing member is required to have the following properties: (1) appropriate elasticity with a low hardness in order to obtain a nip necessary for development, (2) sufficient recovery properties from compression caused by the contact with the electrostatic latent image bearing member, and (3) allowing a layer to be uniformly coated onto the core of the development roller in order to obtain the desired development characteristics, (4) providing the toner particles on the development roller with the desired charge polarity and charge quantity, (5) sufficient releasability from the toner particles so as not to cause the filming phenomenon, (6) appropriate surface roughness for forming a uniformly thin toner layer on the surface of the development roller, (7) sufficient lubricity to reduce the chatter caused by the frictional resistance during the contact development, and (8) wear-resistance.
To meet the above-mentioned requirements, an elastic body having a low hardness, as usually referred to rubber, is conventionally employed as a coating material for the core of the development roller.
Specific examples of such a rubber are polar rubbers such as nitrile--butadiene rubber (NBR), epichlorohydrin rubber (ECO), acrylic rubber, and chloroprene rubber, and high resistance rubbers such as silicone rubber, ethylene--propylene rubber, and styrene--butadiene rubber, in which particles with a low electric resistivity such as carbon and metal powder, serving as resistance adjusting agent, are dispersed. These rubbers are molded into a development roller on a cylindrical core by a conventional molding method such as press-molding and vapor-molding.
However, the conventional development rollers can satisfy not all of the above requirements.
In particular, the above-mentioned requirements (1) through (3) are essential for the contact development method. However, it is difficult to obtain a development roller which satisfies not only requirements (1) to (3), but also the requirements (4) to (8) for the surface properties of the development roller.
This difficulty stems from, for example, the following facts. When the hardness of a rubber used for the development roller is decreased, sufficient surface grinding properties cannot be obtained, resulting in the difficulty in finishing the surface of the rubber with a desired surface roughness (Rz) expressed as a value of the order of less than several .mu.m. In addition, such a rubber has not only a tendency to have a large frictional resistance to the member which comes into contact therewith because of its surface adhesiveness, but also a tendency to easily wear because of its low hardness. When the polar rubber is used for the development roller, toner particles persistently adhere to the surface of the development roller due to its poor releasability from toner, resulting in the toner filming. Accordingly, the charge quantity of the toner is changed, so that the deposition amount of the toner on the development member becomes non-uniform, and accordingly images become non-uniform in quality.